The field of this invention is the art of filtration of particulate solids from a liquid or gas, and particularly filtration operations carried out with semifluidized beds of filtration media. Semifluidization refers to the phenomenon created when a mass of fluidized particles is compressed from above with a porous restraining plate, giving rise to the formation of a fluidized bed and a fixed bed in series within a single containing vessel. The concept of semifluidization was first disclosed by Fan et al, A.I. Ch. E. Journal, Vol. 5, pp. 407-409, Sept. 1959, "Semifluidization". See also Fan et al U.S. Pat. No. 3,374,052. As shown by the cited publications, semifluidized beds were first described as a means for carrying out chemical reactions in which the high reaction rate of a fluidized bed is obtained within the initial fluidized portion of the bed, while the reaction is completed in the final packed section of the bed, taking advantage of the higher completion rate of fixed bed reactors.
Semifluidized beds have also been referred to as "convertible" beds, since the relative size or extent of the fluidized and packed sections are subject to control, the bed being convertible over the range from a full bed fluidization to a completely packed bed, with the relative size and extent of the fluidized and packed sections being selectively variable. The degree of fluidization of a semifluidized bed can be controlled by the position of the upper porous retainer and/or by the fluidization velocity. As the retainer is moved downwardly and/or as the fluidization velocity is increased, the downward extent of the packed section is increased. Thus, processes may be carried out with fluidized and packed bed sections of selected relative size. Further, during a chemical process or physical operation, the extent of the packed section relative to the fluidized section can be increased or decreased as required for promoting the process or operation. See, for example, Wen & Fan U.S. Pat. No. 4,157,959 and Fan & Wen U.S. Pat. No. 4,253,947. Further, as shown by these patents, at the conclusion of a filtration operation or a biological reaction, the upper retainer may be raised to a position at which the bed can be fully fluidized for purpose of removing accumulated solids, such as filtered solids, or bacterial floc.
Wen & Fan U.S. Pat. No. 4,157,959 describes the application of a convertible semifluidized bed of a filtration medium to the removal of fine particulate solids from a liquid or gas. The teachings of that patent are therefore particularly relevant to the subject matter of this application. In the method of the Wen & Fan patent, the filtration operation is started with all, or at least most, of the filter medium in the form of a packed bed. The filtration proceeds with the fluid containing the particulates to be filtered being passed upwardly through the packed bed from the bottom to the top thereof. As with conventional packed bed filters, the solids accumulate in the entry section of the bed, the extent of solids penetration into the bed being limited, and with the formation of a typical filter cake across the lower end of the fixed bed. This causes the usual large increase in pressure drop across the bed, or more particularly the cake portion thereof, so that the filtration operation would normally need to be discontinued as the condition of complete plugging of the entry portion of the filter bed is reached. However, the plugging requiring shutdown is prevented, as described in U.S. Pat. No. 4,157,959, by lifting the upper retaining plate so that the filter cake-containing lower portion of the bed breaks off and becomes fluidized. The filtered solids of the cake are, in effect, resuspended in the fluidized section. Moreover, the motion of the fluidized particles tends to scour the bottom of the fixed bed as the filtration operation continues. This scouring action delays the buildup of the next increment of filter cake. When a second filter cake has accumulated to a point approaching the condition of plugging, the upper retainer is again lifted to again resuspend the filtered solids. This filtering and resuspension process is continued with progressive increase in the size of the fluidized bed portion of the bed and corresponding decrease in the packed bed section. The process may be continued until the packed section approaches the minimal depth for preventing breakthrough of the filtered solids. At that time, the filtration operation is terminated, and the retainer plate is raised to a height permitting the entire bed to be fluidized so that the accumulated solids can be removed prior to the start of the next filtration cycle.
One disadvantage of the filtration method of U.S. Pat. No. 4,157,959 in commercial applications is that means must be provided for staged upward movement of the media retainer plate, and associated controls and instrumentations so to achieve movements at optimum times for maximizing the filtration capacity of the apparatus. For best results the sensing of pressure buildup across the filter cake interface needs to be carefully measured at each stage. Also, as the load of resuspended solids within the fluidized section increases, the rate at which the filter cake develops will be progressively faster. Therefore, the retainer will be moved at increasingly frequent intervals as the filtration cycle proceeds.